To my knowledge, both How could I...? and How could I have...? can be used to express disbelief at something you did in the past or you have just done. What could be the difference, if there is any?
1 Answer
could is for past ability. (We're not using it as a conditional clause.)
How could I do this? (I did it and I'm asking myself this.)
could have is for past possibility.
How could I have done this? (There was a way to do this, but I didn't, so I'm asking myself this.)
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But couldn't we say, "How could I have done something so stupid?" when we really did/have done some insensible act? Commented Nov 27, 2015 at 17:27
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If you did it, you use How could I do something so stupid?– SchwaleCommented Nov 27, 2015 at 18:02